Since this seems to be a big thing for FossaMail users and documentation is spread rather thin on the 'net, here's a howto by yours truly!

FossaMail comes with a calendar add-on as an extension, called "lightning", available for download from the same location as the mail/news/chat client.

Important note:Lightning's on-line integration function was not operational in 25.1.0, due to a discrepancy in the http module when the chat core was upgraded.This should be fixed in 25.1.1! You won't be able to apply the information in this howto in 25.1.0 - please update.

You can integrate with Google Calendar in several ways. they offer iCal support, although they do not allow you to create/edit with it (it's read-only ), you can use CalDAV, or you can use the Google Calendar Provider add-on in tandem with lightning. In this howto, I will focus on the CalDAV, since iCal is very limited in read-only mode, and is straightforward to set up if you want it, from the New Calendar wizard. I may add a section for the Google Calendar Provider add-on if it's needed/desired.

Using CalDAV

How do I set this up?

First, you need to gather some information from Google Calendar, namely the "Calendar ID". If you only use one Google Calendar or just want to synchronize with your primary calendar, this ID will be your google e-mail address.If you use multiple calendars and want to pick a specific non-primary calendar, you need to find your calendar ID on the Google Calendar website: in the pull-down menu next to the calendar name, select Calendar Settings. On the resulting page, the calendar ID is shown in a section labelled Calendar Address (after buttons for html/ical/xml). For secondary calendars this is usually in the form of a_long_hash@group.calendar.google.com - copy this to your clipboard.

Replace CALENDAR_ID with the Calendar ID you have from step 1 (or your gmail address if it's just your primary calendar you want)

Select "offline support" if you want to be able to change your calendar when not connected to Google servers (this may cause a large download if you have a lot of events already stored in your Google Calendar). Keep in mind that working off-line can be tricky.

Click next, and give the calendar a name for in your list and optional color.

Click next again, and you will be prompted to log in to Google, and then give permission to Lightning to access your calendar on-line.

Potential issues:

The Google Login popup (step 8) complains that cookies are not supported.Go to FossaMail options, Security, Web Content, and check "Accept cookies from sites", keeping them "until they expire"